Oil sands are deposits comprised of bitumen, clay, sand and connate water, and make up a significant portion of North America's petroleum reserves. To produce a marketable hydrocarbon product from the oil sands, the bitumen must be extracted from the oil sands matrix. Because the bitumen itself is a tar-like, highly viscous material, separating it from the sands poses certain practical difficulties.
Bitumen Extraction Process:
An example of a common extraction technique is known as a water-based bitumen extraction process or a bitumen froth flotation, where hot water, air, and process aides are added to crushed ore at a basic pH to form a slurry. The slurry is sent to a primary separation vessel (PSV), which typically results in three streams including: (i) a bitumen froth stream, (ii) a coarse tailings stream, and (iii) a middlings stream. The bitumen froth stream is made up of bitumen, water and fine solids, and is formed from the buoyant bitumen-coated air bubbles rising through the slurry. The coarse tailings stream, also known as PSV underflow, is primarily made up of coarse solids (e.g. greater than 44 microns), some fine solids, and water. The coarse tailings stream is formed from the material that settles at the bottom of the PSV, and is discharged from the base of the vessel. A middlings stream, also known as fine tailings stream, may also be collected, comprising non-buoyant bitumen, water, and fine solids (e.g. less than 44 microns), from the middle of the PSV. In general, the middlings stream is subjected to a further froth flotation treatment to generate a secondary bitumen froth that is combined with the primary bitumen froth for further processing. This secondary treatment also generally produces a secondary tailings also known as fine tailings stream.
One problem with the water-based bitumen extraction process is the treatment of the fine tailings produced. The standard industry practice until now has been to pump oil sands tailings into large settling ponds—often discontinued mine pits. There, the heaviest material—mostly sand—settles to the bottom, separating from the top layer of water which can be recycled. The middle layer eventually settles to form what is known as mature fine tailings (MFT) which is comprised of about 70% water and 30% fine clay. Residual heat escapes into the atmosphere, while the tailings water is retained for future use, with some loss due to evaporation. This method is not optimal for at least three reasons. First, a significant amount of time is required for most of the solid materials to settle out of the fine tailings by operation of gravity alone; it can take decades for the fine particles to settle. Second, it does not allow for the recovery of the energy contained within the tailings stream in the form of heat. The fine tailing streams are often at temperatures between 35° C. and 90° C. Third, tailings ponds do not permit recovery of any of the residual hydrocarbon component (e.g. bitumen) within the tailings.
Several attempts to recover heat, water, and other reagents from tailings streams are known. Methods are disclosed in U.S. Pat. Nos. 4,343,691, 4,561,965 and 4,240,897, all to Minkkinen. These patents are directed to heat and water vapor recovery using a humidification/dehumidification cycle. U.S. Pat. No. 6,358,403 to Brown et al. describes a vacuum flash process used to recover hydrocarbon solvents from heated tailings streams. CA 2,674,660 describes the recovery of water from the tailings produced in PFT.
Paraffinic Froth Treatment:
The bitumen froth (i.e. the combination of the primary and secondary froth, discussed above) typically comprises bitumen (approximately 60% by weight), water (approximately 30% by weight), and solids (approximately 10% by weight). The water and solids in the froth are contaminants which need to be reduced in concentration before further treatment in a downstream refinery-type upgrading facility or shipped in a pipeline. This cleaning operation is carried out using what is referred to as “froth treatment”.
Froth treatment is the process of reducing the aqueous and solid contaminants from the froth to produce a clean bitumen product. The froth is first diluted with a hydrocarbon solvent to reduce the viscosity and density of the oil phase, thereby accelerating the settling of the dispersed phase impurities by gravity or centrifugation. One such froth treatment process is known as paraffinic froth treatment (PFT), which involves the introduction of a paraffinic solvent (saturated aliphatic, such as a mixture of n-pentane and iso-pentane) as the hydrocarbon solvent. The bitumen product, called “diluted bitumen”, which is often abbreviated to “dilbit”, is then separated from diluted tailings, comprising water, solids, and some hydrocarbons. The solvent is recovered from the diluted bitumen to be used again.
The tailing from the PFT process (also called paraffinic froth treatment underflow (PFT underflow)) undergoes a further treatment to remove the solvent in a tailings solvent recovery unit (TSRU). A tailings solvent recovery unit (TSRU) utilizes heat to separate the hydrocarbon solvent from the particulate-containing water stream. Often, some of the tailings particulates remain in the TSRU, this accumulation of inorganic and organic solids resulting in the fouling or plugging of vessel internals, lines and valves. To reduce foaming and plugging within the TSRU, hot process water (e.g. 90° C.) (also called dilution water) is added to the PFT underflow before entering the TSRU.
The paraffinic froth treatment process may be carried out at high temperature (approximately 70-90° C.), and is then known as high temperature paraffinic froth treatment (HT-PFT). Paraffinic froth treatment has been discussed in the prior art, such as in Canadian Patent Nos. 2,149,737 (to Tipman and Long), 2,217,300 (to Shelfantook et al.), and 2,587,166 (to Sury).
One problem with the traditional PFT process includes the use of fresh water. This means that fresh water reserves are depleted. Other problems include plugging that occurs in the TSRU.